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1. Synthesis and Li-ion electrode properties of layered MAB phases Ni _n+1 ZnB _n ( n = 1, 2)

   https://doi.org/10.1039/c9ta12937e  

   Rezaie, Amir A. ; Yan, Zheng ; Scheifers, Jan P. ; Zhang, Jian ; Guo, Juchen ; Fokwa, Boniface P. ( January 2020 , Journal of Materials Chemistry A)

   Ternary MAB phases (layered transition metal borides) have recently attracted interest due to their exfoliation potential toward MBenes (analogous to MXenes), which are predicted to have excellent Li-ion battery performance. We have achieved single-phase synthesis of two MAB phases with general composition Ni n+1 ZnB n ( n = 1, 2), the crystal structures of which contain zinc layers sandwiched between thin ( n = 1) and thick ( n = 2) Ni–B slabs. Highly stacked MAB sheets were confirmed by X-ray diffraction and high-resolution scanning electron microscopy for both materials. Exposing Ni n+1 ZnB n to diluted hydrochloric acid led to the creation of crystalline microporous structures for n = 1 and non-porous detached sheets for n = 2. Both morphologies transformed the inactive bulk materials into highly active Li-ion battery anodes with capacities of ∼90 mA h g −1 ( n = 1) and ∼70 mA h g −1 ( n = 2) at a 100 mA g −1 lithiation–delithiation rate. The XPS analysis and BET surface area measurements reveal that the increased surface area and the reversible redox reaction of oxidized nickel species are responsible for this drastic increase of the lithiation–delithiation capacity. This proof of conceptmore »opens new avenues for the development of porous MAB, MAB nanosheets, MBenes and their composites for metal-ion battery applications.« less
2. Probing Charge Transport and Background Doping in Metal‐Organic Chemical Vapor Deposition‐Grown (010) β‐Ga ₂ O ₃

   https://doi.org/10.1002/pssr.202000145  

   Feng, Zixuan ; Bhuiyan, A. F. M. Anhar Uddin ; Xia, Zhanbo ; Moore, Wyatt ; Chen, Zhaoying ; McGlone, Joe F. ; Daughton, David R. ; Arehart, Aaron R. ; Ringel, Steven A. ; Rajan, Siddharth ; et al ( June 2020 , physica status solidi (RRL) – Rapid Research Letters)

   A new record‐high room‐temperature electron Hall mobility (μ_RT = 194 cm² V⁻¹ s⁻¹atn ≈ 8 × 10¹⁵ cm⁻³) for β‐Ga₂O₃is demonstrated in the unintentionally doped thin film grown on (010) semi‐insulating substrate via metal‐organic chemical vapor deposition (MOCVD). A peak electron mobility of ≈9500 cm² V⁻¹ s⁻¹is achieved at 45 K. Further investigation on the transport properties indicates the existence of sheet charges near the epilayer/substrate interface. Si is identified as the primary contributor to the background carrier in both the epilayer and the interface, originating from both surface contamination and growth environment. The pregrowth hydrofluoric acid cleaning of the substrate leads to an obvious decrease in Si impurity both at the interface and in the epilayer. In addition, the effect of the MOCVD growth condition, particularly the chamber pressure, on the Si impurity incorporation is studied. A positive correlation between the background charge concentration and the MOCVD growth pressure is confirmed. It is noteworthy that in a β‐Ga₂O₃film with very low bulk charge concentration, even a reduced sheet charge density plays an important role in the charge transport properties.
3. Oxygen Incorporation in the Molecular Beam Epitaxy Growth of Sc _x Ga _1−x N and Sc _x Al _1−x N

   https://doi.org/10.1002/pssb.201900612  

   Casamento, Joseph ; Xing, Huili Grace ; Jena, Debdeep ( January 2020 , physica status solidi (b))

   Secondary‐ion mass spectrometry (SIMS) is used to determine impurity concentrations of carbon and oxygen in two scandium‐containing nitride semiconductor multilayer heterostructures: Sc_xGa_1−xN/GaN and Sc_xAl_1−xN/AlN grown by molecular beam epitaxy (MBE). In the Sc_xGa_1−xN/GaN heterostructure grown in metal‐rich conditions on GaN–SiC template substrates with Sc contents up to 28 at%, the oxygen concentration is found to be below 1 × 10¹⁹ cm⁻³, with an increase directly correlated with the scandium content. In the Sc_xAl_1−xN–AlN heterostructure grown in nitrogen‐rich conditions on AlN–Al₂O₃template substrates with Sc contents up to 26 at%, the oxygen concentration is found to be between 10¹⁹and 10²¹ cm⁻³, again directly correlated with the Sc content. The increase in oxygen and carbon takes place during the deposition of scandium‐alloyed layers.
4. Co _2‐x Rh _x P Nanoparticles for Overall Water Splitting in Basic Media: Activation by Phase‐Segregation‐Assisted Nanostructuring at the Anode

   https://doi.org/10.1002/cctc.202100483  

   Mutinda, Samuel I. ; Batugedara, Tharanga N. ; Brown, Benjamin ; Brock, Stephanie L. ( August 2021 , ChemCatChem)

   The application of Co_2‐xRh_xP nanoparticles as electrocatalysts for the hydrogen evolution reaction (HER) and overall water splitting in basic media is reported. The experimental design seeks to dilute rhodium with earth‐abundant cobalt as a means to lower the cost of the material and achieve catalytic synergism, as reported for related bimetallic phosphides. The HER activity of Co_2‐xRh_xP is found to be composition‐dependent, with the rhodium‐rich compositions being more active as compared to their cobalt‐rich counterparts, with overpotentials (η) at 10 mA/cm²_geometricof 58.1–63.9 mV vs. 82.1–188.1 mV, respectively. In contrast, Co‐rich Co_2‐xRh_xP nanoparticles are active for the oxygen evolution reaction (OER) process in basic media, with η= 290 mV for x=0.25. A full water electrolysis cell was created using the most active compositions for OER and HER as the anode and cathode, respectively, generating an overall η= 390 mV. Notably, the cell became more active over a 50 h stability test, increasing by 2 mV/cm²_geometricat a constant applied voltage of 1.62 V vs NHE. This enhanced activity correlates with nanoscale phase segregation of Rh in the anode. Thus, the lower overpotential achieved for Co_1.75Rh_0.25P relative to Co₂P, and the augmented activity over time in the former, may be a consequence of restructuring of the anode driven by Rhmore »phase‐segregation. The augmentation in activity at the anode more than compensates for small losses at the cathode.

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5. Unexpected Correlation Between Boron Chain Condensation and Hydrogen Evolution Reaction (HER) Activity in Highly Active Vanadium Borides: Enabling Predictions

   https://doi.org/10.1002/ange.202000154  

   Lee, Eunsoo ; Park, Hyounmyung ; Joo, Hyunkeun ; Fokwa, Boniface P. T. ( May 2020 , Angewandte Chemie)

   Transition‐metal borides (TMBs) have recently attracted attention as excellent hydrogen evolution (HER) electrocatalysts in bulk crystalline materials. Herein, we show for the first time that VB and V₃B₄have high electrocatalytic HER activity. Furthermore, we show that the HER activity (in 0.5 mH₂SO₄) increases with increasing boron chain condensation in vanadium borides: Using a −23 mV overpotential decrement derived from −0.296 mV (for VB at −10 mA cm⁻²current density) and −0.273 mV (for V₃B₄) we accurately predict the overpotential of VB₂(−0.204 mV) as well as that of unstudied V₂B₃(−0.250 mV) and hypothetical “V₅B₈” (−0.227 mV). We then derived an exponential equation that predicts the overpotentials of known and hypothetical V_xB_yphases containing at least a boron chain. These results provide a direct correlation between crystal structure and HER activity, thus paving the way for the design of even better electrocatalytic materials through structure–activity relationships.